Recently, as the operating frequency and efficiency of computers, peripheral devices, and many other electronic apparatuses have been continuously increased, designers of DC-to-DC converters have made great effort in reducing the power loss of such converters as much as possible, in order to provide the aforesaid electronic apparatuses with a DC input voltage that features high efficiency, high reliability, and a highly flexible range. Therefore, resonant converters based on the soft switching technique have emerged which operate on the following principle. First of all, a resonant inductor, a resonant capacitor, and like elements are provided on the primary side of a transformer via series connection, parallel connection, or series-parallel connection. Then, by means of a resonant control chip and the principle of resonance, zero-voltage or zero-current switching of the power elements in the resonant converter is achieved to effectively reduce the switching loss of the power elements, thereby increasing the overall conversion efficiency. Nowadays, with the advancement of the manufacturing technology of resonant control chips and power elements, plus the gradually declining prices of such components, resonant converters with high efficiency, high operating frequency, and a simple structure have become more and more popular in the industry and are extensively used in a variety of electronic appliances. In particular, series resonant converters are preferred industry-wide for their high efficiency and wide output voltage range at high input voltage.
Referring to FIG. 1, a series resonant converter commonly used in the industry typically includes an input voltage filter capacitor Cin; a resonant control chip IC; a first power switch Q1; a second power switch Q2; a resonant inductor Lr; a resonant capacitor Cr; a transformer T1; two secondary rectifier diodes D1, D2; and an output voltage filter capacitor Cout. The input voltage filter capacitor Cin is connected across the positive and negative ends of a DC input voltage V. The first power switch Q1 and the second power switches Q2 are connected in series to each other and are connected in parallel to the input voltage filter capacitor Cin. The gates of the first and second power switches Q1, Q2 are connected to the corresponding control pins of the resonant control chip IC respectively. For example, if the resonant control chip IC is the high-voltage resonant controller ST L6599A made by the famous chip manufacturer STMicroelectronics, the gates of the first and second power switches Q1, Q2 will be connected to the control pins HVG, LVG of the resonant control chip IC respectively. The drain and the source of the first power switch Q1 are connected to the anode of the input voltage filter capacitor Cin, and the drain of the second power switch Q2 respectively. The source of the second power switch Q2 is connected to the cathode of the input voltage filter capacitor Cin. Thus, the input voltage filter capacitor C1 is capable of providing a stable input voltage to the transformer T1. The transformer T1 is configured mainly for isolation and includes a primary winding NP and two secondary windings NS1, NS2. The primary winding NP has one end connected to the anode of the resonant capacitor Cr and the other end connected via the resonant inductor Lr to a line between the two power switches Q1, Q2. Meanwhile, the cathode of the resonant capacitor Cr is connected to the source of the second power switch Q2. The secondary winding NS1 has one end connected to the anode of the output voltage filter capacitor Cout and the other end connected to the negative end of the secondary rectifier diode D1. Similarly, the secondary winding NS2 has one end connected to the anode of the output voltage filter capacitor Cout and the other end connected to the negative end of the secondary rectifier diode D2. The positive ends of the secondary rectifier diodes D1, D2 are connected to the cathode of the output voltage filter capacitor Cout. Thus, the output voltage filter capacitor Cout is capable of providing a stable DC output voltage Vout to a load connected across the output ends. The working principle of such a conventional series resonant converter is briefly stated as follows. By virtue of the impedance properties of the resonant inductor Lr and the resonant capacitor Cr series-connected on the primary side, the resonant control chip IC controls the switching frequencies of the two power switches Q1, Q2 and thereby enables the series resonant converter to provide a stable output voltage according to the load connected across the output ends.
FIG. 2 shows the results of using a probe with an internal resistance of 100 MΩ to measure the voltage of the resonant capacitor Cr when the conventional series resonant converter described above is turned off. In practice, due to the load effect of the probe during measurement, the high voltage Vcr accumulated at the resonant capacitor Cr is discharged only gradually but is not reduced to zero instantly. More specifically, it takes at least about 7 seconds (i.e., the period indicated by >7 S in FIG. 2) for the voltage Vcr of the resonant capacitor Cr to go to zero. On the other hand, the horizontal dashed line in FIG. 2 represents the voltage waveform of the resonant capacitor Cr in the absence of the load effect of the probe. As the high voltage Vcr accumulated at the resonant capacitor Cr finds no discharge path under such a condition, the voltage of the resonant capacitor Cr stays above 0.5 Vbus at power-off. If Vbus=400V, the voltage of the resonant capacitor Cr will be 200V or above. Therefore, immediately when the series resonant converter is turned on from off, as shown in FIG. 3, the high voltage stored in the resonant capacitor Cr generates such a high inrush current that, upon switching the second power switch Q2, the voltage Vds across the drain and the source of the second power switch Q2 reaches a peak voltage Vpk of 540V (as indicated by the upper left arrow in FIG. 3). Meantime, a peak current Ippk as high as 20 A is generated on the primary side (as indicated by the arrow corresponding to the primary-side current Ip in FIG. 3), and a peak current Ispk as high as 280 A is generated on the secondary side (as indicated by the arrow corresponding to the secondary-side current Is in FIG. 3). As a result, both the primary side and the secondary side require power switches of high rated voltages and high rated currents. Therefore, the issue to be addressed by the present invention is to design and make a series resonant converter which not only has a simple circuit composed of low-cost electronic components, but also can zero the voltage of the resonant capacitor Cr instantaneously at power-off so that, when the series resonant converter is turned on from off, the inrush current of the resonant capacitor Cr will be low, thereby effectively reducing the peak voltage Vpk generated at the second power switch Q2, the peak current Ippk on the primary side, and the peak current Ispk on the secondary side.